U.S. patent application number 11/314558 was filed with the patent office on 2006-05-18 for process and device for analysis of radioactive objects.
Invention is credited to Abdallah Lyoussi, Raymond Pasquali-Barthelemy, Emmanuel Payan, Anne-Cecile Raoux.
Application Number | 20060104400 11/314558 |
Document ID | / |
Family ID | 9544170 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060104400 |
Kind Code |
A1 |
Lyoussi; Abdallah ; et
al. |
May 18, 2006 |
Process and device for analysis of radioactive objects
Abstract
According to the invention, an object (2) and particularly a
radioactive waste package that may contain fissile isotopes and/or
fertile isotopes, is analyzed by irradiating the object by thermal,
epithermal and fast neutrons resulting from a series of initial
fast neutron pulses, the prompt and delayed neutron signals emitted
by the object after each pulse are measured, these signals are
accumulated, and the contribution Sp of prompt neutrons originating
form thermal fission and the contribution Sr of delayed neutrons
originating from thermal, epithermal and fast fission are
determined from this sum of all signals, and the quantity of each
isotope is determined using Sp and Sr and additional information
about the isotope quantites.
Inventors: |
Lyoussi; Abdallah;
(Manosque, FR) ; Pasquali-Barthelemy; Raymond;
(Saint Crepin, FR) ; Payan; Emmanuel; (Manosque,
FR) ; Raoux; Anne-Cecile; (Condat, FR) |
Correspondence
Address: |
PEARNE & GORDON LLP
1801 EAST 9TH STREET
SUITE 1200
CLEVELAND
OH
44114-3108
US
|
Family ID: |
9544170 |
Appl. No.: |
11/314558 |
Filed: |
December 21, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10872317 |
Jun 18, 2004 |
|
|
|
11314558 |
Dec 21, 2005 |
|
|
|
09719116 |
Apr 9, 2001 |
|
|
|
PCT/FR00/00848 |
Apr 5, 2000 |
|
|
|
10872317 |
Jun 18, 2004 |
|
|
|
Current U.S.
Class: |
376/159 |
Current CPC
Class: |
G01N 23/222 20130101;
G01T 3/00 20130101; G01T 3/001 20130101 |
Class at
Publication: |
376/159 |
International
Class: |
G21G 1/06 20060101
G21G001/06 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 8, 1999 |
FR |
99 04396 |
Claims
1. Device for analyzing an object (2) that contains fissile
material and fertile material, the fissile material comprising M
fissile isotopes and the fertile material comprising N fertile
isotopes, where M and N are integer numbers equal to at least 1,
said device comprising: means (8, 10) for irradiating the object by
a neutron flux consisting of thermal, epithermal and fast neutrons,
the thermal neutrons being effective to cause fissions in the
fissile material and the epithermal and fast neutrons being
effective to cause fissions in the fissile material and in the
fertile material, said means for irradiating consisting essentially
of one or more sources of fast neutrons operable in pulsed mode and
means of thermalizing fast neutrons supplied by said one or more
sources of fast neutrons to provide said neutron flux consisting of
said thermal, epithermal and fast neutrons, means (4, 52) for
counting neutrons and for measuring prompt and delayed neutronic
signals emitted by the object after each pulse of fast neutrons
supplied from said one or more fast neutron sources, and means (6)
for accumulating measured prompt and delayed neutronic signals and
for obtaining the sum of all such signals, and for determining the
contribution Sp of prompt neutrons produced by the thermal fissions
and the contribution Sr of delayed neutrons produced by the
thermal, epithermal and fast fissions.
2. Device according to claim 1, in which the thermalization means
comprises a containment (10) that includes a central area (12) in
which the object (2) will be placed and in which at least three
sides are delimited by a thickness (14, 60) of moderator material,
the fast neutron source (8) being placed in a fourth side of this
containment and the neutron counting means (4, 52) being placed on
the three sides between the central area and the thickness of
moderator material, a thickness of neutron multiplier material (22,
24, 50) being provided between the central area and the fast
neutron source and between the central area and neutron counting
means.
3. Device according to claim 2, in which each neutron counting
means is also surrounded by a thickness (26) of neutron poison
material.
4. Device according to claim 2, in which each neutron counting
means is also surrounded by a moderator material (28).
5. Device according to claim 2, also comprising a wall (36) made of
neutron poison and moderator materials that delimits the fourth
side of the containment, a corresponding thickness (223) of the
multiplier material being between this wall (36) and the central
area (12).
6. Device according to claim 2, also comprising means (46, 48, 68,
70, 72) of rotating the object (2) within the central area of the
containment.
7. A device according to claim 1, in which the means of
thermalizing comprises a containment that includes a central area
in which the object will be placed and in which at least three
sides are delimited by a thickness of moderator material, the fast
neutron source being placed in a fourth side of this containment
and the neutron counting means being placed between the central
area and the thickness of moderator material, a thickness of
neutron multiplier material being provided between the central area
and the fast neutron source, the device also comprising a wall at
least made of a neutron poison material that delimits the fourth
side of the containment, a corresponding thickness of the
multiplier material being between this wall and the central
area.
8. A device according to claim 1, wherein M equals 3 and N equals
1.
9. A device according to claim 8, wherein the 3 fissile isotopes
are uranium 233, uranium 235 and plutonium 239 and the fertile
isotope is uranium 238.
10. A device according to claim 2 wherein the neutron multiplier
material is Pb.
11. A. device according to claim 1, further comprising means for
determining the quantity of each of the M+N isotopes from the
contribution Sp, the contribution Sr and at least M+N-2 additional
items of information related to the quantities of the M+N
isotopes.
12. A device according to claim 11, said combination Sp and
combination Sr being expressed as linear combinations of the
quantities of said M+N isotopes, wherein coefficients of said
linear combinations are determinable beforehand by calibration.
13. A device according to claim 1, said object being a radioactive
waste package.
14. A device according to claim 8, wherein the 3 fissile isotopes
are uranium 235, plutonium 239 and plutonium 241 and the fertile
isotope is uranium 238.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/872,317 filed Jun. 18, 2004, now pending,
which is a continuation of U.S. patent application Ser. No.
09/719,116 filed Apr. 9, 2001, which is a .sctn.371 of
PCT/FR2000/00848 filed Apr. 5, 2000.
TECHNICAL FIELD
[0002] This invention relates to a process and a device for
analyzing radioactive objects that use a neutronic measurement of
these objects.
[0003] This invention can be used to analyze these objects
non-destructively (in other words without affecting the physical
integrity of the objects) by making active measurements (in other
words controlled by external radiation) on these objects.
[0004] In particular, the invention is applicable to control of the
radioactive product treatment process and charaterization of the
contents of radioactive waste packages. These packages are
containers, usually made of concrete or steel, in which radioactive
waste, possibly previously coated in a matrix, is placed.
[0005] The invention is particularly applicable to the analysis of
the fissile material and/or fertile material contained in these
radioactive waste packages in order to non-destructively determine
the quantities of some chemical elements present in this waste.
[0006] It is applied directly in installations using active
non-destructive analysis techniques. In particular, the analysis of
the fissile material and/or the fertile material is a means of
quantifying the mass of residual fuel.
STATE OF PRIOR ART
[0007] Several measurement methods have been studied in order to
non-destructively determine the quantity of some fissile isotopes
contained in a waste package, including the neutronic interrogation
technique by means of 14 MeV neutrons produced by an appropriate
generator.
[0008] More particularly, measurement of prompt neutrons and
delayed neutrons produced by thermal fission of the fissile
material present in the waste package, is described in document
U.S. Pat. No. 4,483,816 (J. T. Caldwell et al).
[0009] In general, interrogation of an object by a pulsed flux of
thermal neutrons is used to identify the presence of fissile
material within this object. This type of method is usually used to
measure fissile isotopes, namely uranium 235, plutonium 239 and
plutonium 241. However, interpretation of the measurements requires
prior knowledge of the isotopic composition of the fissile
material.
[0010] With the technique described in the document mentioned
above, the main fissile isotopes thus characterized are uranium
233, uranium 235 and plutonium 239. The various isotopes are
quantified by the use of prompt and delayed signals originating
from thermal neutrons. Two linear equations are then obtained. A
third equation is obtained by measuring coincidences on passive
neutrons (in other words neutrons emitted naturally by the
material). Therefore, it is possible to calculate the various
masses of fissile isotopes mentioned above, present in the object
to be measured, provided that several calibration coefficients
(previously calculated) are known.
[0011] Nevertheless, this technique does not give any information
about the presence and quantity of fertile material such as uranium
238 in the object to be analyzed.
DESCRIPTION OF THE INVENTION
[0012] The purpose of this invention is to correct this
disadvantage.
[0013] The characterization of fissile and fertile materials
requires the use of an interrogating flux of thermal, epithermal
and fast neutrons, since the fission threshold of uranium 238 is
located at an energy of about 1 MeV. Furthermore, the contribution
of uranium 238 to the measured neutronic signal can only be used
for delayed neutrons emitted by fission fragments of uranium 238.
Thus, the measured prompt signal corresponds to neutrons produced
by thermal fission (fissile material) and the delayed signal
corresponds to neutrons produced by thermal and fast fission
(fissile and fertile materials).
[0014] This invention combines thermal, epithermal and fast
interrogation with detection of prompt and delayed neutrons in
order to characterize the fissile and/or fertile material that
could be present in an object to be measured.
[0015] More precisely, this invention relates to a process for
analyzing an object, particularly a radioactive waste package, that
might contain a fissile material or a fertile material or both, the
fissile material comprising M fissile isotopes and the fertile
material comprising N fertile isotopes, where M and N are integer
numbers equal to at least 1, this process being characterized in
that: [0016] the object is irradiated by a neutron flux formed of
thermal, epithermal and fast neutrons and resulting from a sequence
of initial pulses of fast neutrons, the thermal neutrons causing
fission in the fissile material and the epithermal and fast
neutrons causing fission in the fissile material and in the fertile
material, [0017] the prompt and delayed neutronic signals emitted
by the object after each pulse are measured, and these signals are
accumulated to obtain the sum of all signals after the last pulse,
[0018] this sum is used to determine the contribution Sp of prompt
neutrons produced by thermal fission and the contribution Sr of
delayed neutrons produced by thermal, epithermal and fast fissions,
[0019] Sp and Sr are expressed as linear combinations 0o the
quantities of M+N isotopes, the coefficients of these linear
combinations being previously determined by calibration, and [0020]
the quantity of each of the M+N isotopes is determined from Sp and
Sr thus expressed and at least M+N-2 additional items of
information about quantities of M+N isotopes.
[0021] For example, this additional information may consist of
correlations between the quantities of M+N isotopes.
[0022] According to one particular embodiment of the process
according to the invention, the fissile and fertile materials
contain uranium 235, uranium 238, plutonium 239 and plutonium
241.
[0023] This invention also relates to a device for analyzing an
object, particularly a radioactive waste package, that may contain
fissile material or fertile material or both, the fissile material
containing M fissile isotopes and the fertile material containing N
fertile isotopes, where M and N are integer numbers equal to at
least 1, this device being characterized in that it comprises:
[0024] means of irradiating the object by a neutron flux consisting
of thermal, epithermal and fast neutrons and resulting from a
sequence of initial fast neutron pulses, the thermal neutrons
causing fission in the fissile material and the enithermal and fast
neutrons causing fission in the fissile material and in the fertile
material, [0025] means of counting neutrons, designed to measure
prompt and delayed neutronic signals emitted by the object after
each pulse, and [0026] means of processing the signals thus
measured, designed to accumulate these signals and, after the last
pulse, to obtain the sum of all the signals, to use this sum to
determine the contribution Sp of prompt neutrons produced by
thermal fission and the contribution Sr of delayed neutrons
produced by thermal, epithermal and fast fission, and to use Sp and
Sr to determine the quantity of each of the M+N isotopes and at
least M+N-2 additional items of information related to the
quantities of M+N isotopes, expressing Sp and Sr as linear
combinations of these quantities, the coefficients of these linear
combinations being determined beforehand by calibration.
[0027] According to a preferred embodiment of the device according
to the invention, the irradiation means comprise: [0028] at least
one source of fast neutrons operating in pulsed mode and, [0029]
means of thermalizing these fast neutrons.
[0030] Preferably, the thermalization means comprise a containment
that includes a central area in which the object will be placed and
in which at least three sides are delimited by a thickness of
moderator material, the neutron source being placed on a fourth
side of this containment and the neutron counting means being
placed on the three sides between the central area and the
thickness of moderator material, a thickness of the multiplier
material being provided between the central area and the neutron
source and between the central area and neutron counting means.
[0031] Each neutron counting means may also be surrounded by a
thickness of neutron poison material.
[0032] Each neutron counting means may also be surrounded by a
moderator material.
[0033] The device according to the invention may also comprise a
wall made of neutron poison and moderator materials that delimits
the fourth side of the containment, the thickness corresponding to
the multiplier material being between this wall and the central
area.
[0034] The device according to the invention may also comprise
means of rotating the object within the central area of the
containment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] This invention will be better understood after reading the
description of example embodiments given below, which are given for
guidance only and are in no way restrictive, with reference to the
attached drawings in which:
[0036] FIG. 1 diagrammatically illustrates the steps in a process
according to the invention,
[0037] FIG. 2 is a diagrammatic cutaway perspective view of a
particular embodiment of the device according to the invention in
an open position
[0038] FIG. 3 is a diagrammatic sectional top view of the device in
FIG. 2 in a closed position,
[0039] FIG. 4 is a diagrammatic perspective sectional view of
another particular embodiment of the invention, and,
[0040] FIG. 5 is a diagrammatic sectional top view of the device in
FIG. 4.
DETAILED PRESENTATION OF PARTICULAR EMBODIMENT
[0041] A process according to the invention uses a thermal,
epithermal and fast interrogating neutron flux in order to provoke
fission reactions in an object that may contain a fissile material
or a fertile material or both. This neutron flux may be obtained
using at least one neutron generator operating in pulsed mode and
producing fast neutrons, for example with an energy of about 14
MeV, for example using the D-T fusion reaction. An adapted
thermalization cell is used to obtain a thermal, epithermal and
fast neutron flux. Firstly, the thermal neutrons provoke fission
reactions in the missile material, and secondly epithermal and fast
neutrons cause fission reactions in the fissile material and in the
fertile material.
[0042] Furthermore, the use of a measurement method in which a
signal is summated after each neutron pulse, is a means of
distinguishing the contribution of prompt neutrons produced by
thermal fission and the contribution of delayed neutrons produced
by thermal, epithermal and fast fission, on the same signal. Only
thermal fission contributes to the prompt signal since eqithermal
and fast fission reactions are instantaneous, therefore their
contribution is drowned in the part of the signal corresponding to
interrogating neutrons.
[0043] Note that more than one pulsed neutron source can be used to
increase the neutron flux and therefore the sensitivity of the
measurements.
[0044] The number of fast neutron pulses may be very large and for
example equal to several million. This depends on the required
precision and detection limit.
[0045] The principle of a process according to the invention using
a pulsed source of fast neutrons and a sequential measurement, is
illustrated diagrammatically in FIG. 1.
[0046] Therefore, the object to be analyzed, for example a
radioactive waste package, is irradiated by thermal, epithermal and
fast neutrons produced by pulses from the source (and obtained as
will be seen later in the description of FIGS. 2 to 5).
[0047] FIG. 1 shows the time t on the abscissa and the number of
counts per second C(s.sup.-1) on the ordinate (on a logarithmic
scale).
[0048] Neutron pulses I1 (first pulse), I2, I3, . . . , In-1 and In
(last pulse) are shown in the figure. The period of the generator
is denoted T. The end of the last pulse occurs at an instant
denoted Ti. The signal due to a single pulse denoted S1 can also be
seen, together with the integrated signals due to two pulses (S2),
three pulses (S3), . . . n-1 pulses (Sn-1) and n pulses (Sn).
[0049] Therefore, the prompt neutron signals such as sp and delayed
neutron signals such as sr emitted after each source pulse are
measured, and these signals are accumulated. The contribution Sp of
prompt neutrons produced by thermal fission and the contribution Sr
of delayed neutrons produced by thermal, epithermal and fast
fission, are determined from the integrated signal Sn.
[0050] Thus, two items of information Sp and Sr about the residual
fuel located in the package can be determined in a single
measurement.
[0051] According to the invention, these results are coupled with
at least two other items of information, for example such as
correlations relating the required isotope masses and obtained by
calculation programs associated with operating experience in fuel
reprocessing plants.
[0052] For example, it is assumed that the package contains
residual uranium 235, uranium 238, plutonium 239 and plutonium 241.
All this information could then written, for example in the form of
the following system of equations: S p = a 1 .times. m .times.
.times. ( 235 .times. U ) + a 2 .times. m .times. .times. ( 238
.times. U ) + a 3 .times. m .times. .times. ( 239 .times. PU ) + a
4 .times. m .times. .times. ( 241 .times. Pu ) Sr = b 1 .times. m
.times. .times. ( 235 .times. U ) + b 2 .times. m .times. .times. (
238 .times. U ) + b 3 .times. m .times. .times. ( 239 .times. PU )
+ b 4 .times. m .times. .times. ( 241 .times. Pu ) R 1 = m .times.
.times. ( 235 .times. U ) m .times. .times. ( 238 .times. U ) R 2 =
m .times. .times. ( 241 .times. Pu ) m .times. .times. ( 239
.times. Pu ) ##EQU1## where: [0053] Sp=signal generated by prompt
neutrons produced by thermal fission (count/sec.), [0054] Sr=signal
generated by delayed neutrons produced by thermal, epithermal and
fast fission (count/sec.), [0055] R.sub.1=correlation between the
mass m(.sup.215U) of the uranium 235 isotope and the mass
m(.sup.238U) of the uranium 238 isotope. [0056] R.sub.2=correlation
between the mass m(.sup.239U) of the plutonium 239 isotope and the
mass m(.sup.241U) of the plutonium 241 isotope. [0057] a.sub.i and
b.sub.i (where i varies from 1 to 4): calibration coefficients in
count.s.sup.-1.g.sup.-1, obtained with a known object (the masses
being expressed in grams).
[0058] The calibration coefficient a.sub.2 is zero since the
fertile material, in the event .sup.238U, does not participate in
the measured signal generated by prompt neutrons.
[0059] Solution of this system gives the required masses.
[0060] The advantage of this process conform with the invention is
due to the fact that the fissile material and the fertile material
present in the object to be measured can be "interrogated"
simultaneously making use of one or several pulsed sources of fast
neutrons, for example 1 or several pulsed generators of 14 MeV
neutrons.
[0061] Due to its design (examples will be given later), the device
used to implement this process can produce a thermal, epithermal
and fast flux while amplifying the fast component.
[0062] The contrast between the fissile material and the fertile
material is thus improved.
[0063] Furthermore, the use of an associated sequential acquisition
method significantly improves the sensitivity of the measurement of
the delayed signal, thus overcoming the poor statistics of delayed
fission neutrons. Furthermore, the combination of additional
information, for example such as correlations of the different
searched isotopes involving mass, molar, atomic or other ratios, is
a means of separately quantifying each of the fissile and fertile
isotopes present in the waste. Therefore this quantification of
each isotope is obtained following a single and unique neutronic
measurement on the analyzed object.
[0064] The device according to the invention as shown in the
cutaway perspective view in FIG. 2, and in the sectional top view
in FIG. 3, is designed to characterize an object, for example a
radioactive waste container 2.
[0065] This device comprises: [0066] means of irradiating the
container 2 by a thermal, epithermal and fast neutron flux, [0067]
neutron counting means 4 in order to measure prompt and delayed
neutron signals emitted by the container after each pulse and,
[0068] signal processing means 6 to process the signals thus
measured in order to accumulate these signals, and to use the sum
of these signals to determine the contribution Sp of prompt
neutrons produced by thermal fission and the contribution Sr of
delayed neutrons produced by thermal, epithermal and fast fission,
and to determine the mass of each of the fissile and fertile
isotopes of the waste as seen above.
[0069] The irradiation means comprise a fast neutron generator 8
operating in pulsed mode and a thermalization containment 10 for
these fast neutrons in order to obtain the thermal, epithermal and
fast neutron flux.
[0070] This containment comprises a central area 12 in which the
container 2 will be fitted. The shape of this central area is
approximately square and it is delimited by a wall 14 made of a
moderator material, for example graphite.
[0071] Part 16 of this wall is mobile--for example it is installed
on rails as shown in FIG. 2--so that the container can be inserted
in the central area.
[0072] FIG. 2 shows that the containment is open whereas it is
closed in FIG. 3 (when the container is irradiated by
neutrons).
[0073] The part of the wall 14 facing the mobile part 16 comprises
a space 20 in which the neutron generator 8 is housed.
[0074] The neutron count means are neutronic detection blocks 4
installed in the mobile part 16 of the wall 14 and in the two parts
of the wall that are adjacent to this mobile part and are facing
each other.
[0075] An element 22 made of a multiplier material, for example
lead, is inserted between the generator and the central area 12.
Similarly, another element 24 made of this multiplier material is
inserted between each group of detection blocks 4 and this central
area.
[0076] Furthermore, each detection block 4 is surrounded by a layer
26 of neutron poison material, for example such as cadmium, and
contains neutron counters, for example .sup.3He detectors
surrounded by another moderator material 28, for example
polyethylene.
[0077] The containment is closed at its upper part by a graphite
cover 30. It is closed at its lower part by a bottom 32 also made
of graphite. This containment is also supported on a base 34, for
example made of steel.
[0078] The device in FIG. 2 also comprises a wall 36 free to move
on rails 38 fitted on base 34 so that it can be moved towards or
away from the part of the wall 14 at which the generator 8 is
located. This mobile wall 36 is separated from this part in the
case shown in FIG. 2, whereas it is in contact with this part in
the case shown in FIG. 3.
[0079] This mobile part 36 is made of neutron poison and moderator
materials; for example, it may be composed of an element 40 made of
graphite, coated with a boron carbide layer 42 facing the part of
the wall 14 on which the generator is located.
[0080] Note that the Last neutrons emitted by the generator 8
towards the mobile wall 36 are thermalized by the graphite element
40 and are absorbed by the boron carbide layer 42 and therefore do
not return to the container 2. This mobile wall 36 can be used to
adjust the neutron flux.
[0081] Means of rotating this container within the central area of
the containment may be provided (FIG. 2) in order to obtain uniform
irradiation of the container 2 by neutrons. These rotation means
may comprise a plate (not shown) on which the container is
supported and means of rotating the plate, for example comprising a
shaft 44 rigidly fixed to this plate and passing through the bottom
32 of the containment 10, and another shaft 46 rotated by a motor
not shown and rotating the shaft 44 by means of gears contained in
a box 48.
[0082] The block detectors 4 that are used to count the prompt
signal and the delayed fission signal are preferably optimized in a
known manner to optimize the sensitivity at a given energy.
[0083] Obviously, they are connected to electrical power supplies
(not shown) necessary for their operation, and are also connected
to signal processing means 6 located outside the containment
10.
[0084] The lead elements 24 that are placed in front of detection
blocks 4 have a radiological shielding function. The measured
containers may be very radioactive and in particular may emit high
gamma radiation. It is then necessary to protect the counters so
that they can be used under optimum conditions.
[0085] Neutrons output from the generator 8 enter into the lead
elements 22 and 24, and reactions of the (n, 2n) type are applied
to them. This can increase the intensity of the interrogating
neutron flux by about 60%.
[0086] Each interrogating neutron can then interact in two possible
ways:
[0087] 1) The neutron is sufficiently slowed by the moderator
materials, the materials in the structures and the object to be
measured itself, until they reach thermal energy. It then induces
fission reactions on the fissile material (for example .sup.235U,
.sup.239Pu, .sup.241Pu) in the object to be measured.
[0088] 2) The neutron is slowed but its energy is higher than about
1 MeV. It then induces fission reactions in the object to be
measured, not only on the fissile material (for example .sup.235U,
.sup.239Pu, .sup.241Pu), but also on the fertile material (for
example .sup.238U).
[0089] Following thermal fission, several fast neutrons (on average
2 to 3 per fission) with an average energy of 2 MeV are emitted
instantaneously; these are the prompt neutrons. They are detected
in blocks 4 surrounded by neutron poison material, such as cadmium,
that absorbs neutrons and makes them sensitive only to fast
neutrons. This is a means of eliminating most of the background
noise due to neutrons produced by the generator 8, that are thermal
at this time of the measurement. However, the prompt neutrons
signal is superposed on different background noise terms, the main
terms being the "active background noise" (active signal without
the contaminant) and the background noise due to passive neutron
emission from the contaminant.
[0090] The measurement of prompt neutrons cannot start unit the
neutrons in the generator have been fully thermalized, since the
signal that they induce during a few hundred microseconds after the
generator pulse is very high. Consequently, all prompt neutrons
produced during this first measurement phase, and particularly
neutrons produced by fission reactions induced by fast neutrons
from the generator, cannot be detected since they are drowned in
the background noise.
[0091] The signal due to delayed fission neutrons is superposed on
different background noises, the most important of which is the
passive neutronic emission from the contaminant. The signal from
the delayed neutrons appears to be constant during the scale of a
measurement cycle, with a duration of about 10 ms, since their
emission time is very long compared with this duration. They start
a few hundred milliseconds to several tens of seconds after the
fission reaction from which they originate following the
.beta.-disintegration of some fission products. Therefore, detected
delayed neutrons originated from previous measurement cycles.
[0092] Delayed neutrons produced by fission reactions induced by
fast neutrons contribute to the delayed neutron signal. Since the
emission of a delayed neutron is delayed after the fission reaction
that generated it, it is possible to detect delayed neutrons
produced by fission reactions induced by fast or epithermal
neutrons, or by thermal neutrons.
[0093] One important consequence is that the fertile material (for
example .sup.238U) contributes to the delayed neutrons signal, but
not to the prompt neutrons signal, since prompt neutrons
originating from fast or epithermal fission reactions are not
detectable. The effective fission cross section of this isotope at
thermal energy is very small compared with the cross section of
fissile isotopes, which makes its contribution to the prompt
neutrons signal completely negligible since the energy spectrum of
the interrogating neutrons is purely thermal during the prompt
neutrons measurement.
[0094] However, the efficient fission cross section of uranium 238
is of the same order of magnitude as the fission cross section of
fissile isotopes beyond 1 MeV. Furthermore, since this isotope may
sometimes be present in large proportions in the contaminant, it
induces a delayed signal that is not negligible compared with the
signal due to fissile isotopes.
[0095] A sequential count method is used during acquisition of the
signal. Thus, information originating from the contributions of
fast and delayed neutrons to the total signal, for example
associated with correlations such as the mass ratios of uranium
isotopes 235 and 238 and plutonium isotopes 239 and 241, can be
used to quantify each of the isotopes mentioned above.
[0096] Another device according to the invention is shown
diagrammatically in FIGS. 4 and 5. FIG. 4 shows a perspective
sectional view of this other device whereas FIG. 5 shows a top
sectional view.
[0097] The device shown in FIGS. 4 and 5 also includes a
containment 10 comprising a central area 12 that for example will
receive a radioactive waste container 2 and is delimited by four
walls 50 made of a multiplier material, for example such as
lead.
[0098] Neutron counters 52 are placed outside three of these walls
and adjacent to these walls, and are surrounded by a moderator
material, for example polyethylene. Two pulsed fast neutron
generators 8 are placed outside the fourth wall 50 and adjacent to
it.
[0099] As will be seen in FIG. 5, walls 54 made of a moderator
material, for example graphite, are placed in contact with the
neutron counters.
[0100] Elements 58 made of an absorbent material, for example
borated polyethylene, cover the surfaces of the assembly thus
obtained except for the surface on which the neutron generators are
located. Furthermore, elements 60 made of a moderator material, for
example polyethylene, cover the elements 58 made of an absorbent
material.
[0101] FIG. 5 also shows the signal processing means 6 that process
signals output by neutron counters 52.
[0102] Layers (not shown) of a neutron poison material, for example
cadmium, cover the neutron detectors.
[0103] A sealing layer 62, for example made of a plastic material,
surrounds the walls 50.
[0104] FIG. 4 shows the base 64 of the containment, which may for
example be made of steel. It also shows various thicknesses of
concrete 66 surrounding the device.
[0105] Means of rotating the container may also be provided, for
example comprising the rotating plate 68 that can be rotated by
means of an appropriate mechanism 70, though a shaft 72 passing
through the base 64.
[0106] The upper part of the device in FIGS. 4 and 5 is covered by
a steel plate 74. This plate is provided with an opening facing the
central area of the containment. This opening is used to place
container 2 in this area, and to take it out of the device after
the measurements. Furthermore, this opening is closed by a cover
76, for example made of steel, fitted with a gripping system 78.
This cover is extended downwards by an element 80 made of a
moderator material, for example polyethylene.
[0107] FIG. 4 also shows a fixed wall 82 made of concrete that is
located facing the neutron generators 8 and that is separated from
them by a space. The face of this wall 82 that is opposite the
generators is fixed to a flux monitor 84 designed to determine the
number of neutrons emitted by the two neutron generators 8.
[0108] Appropriate means (not shown) may be provided opposite the
other face of the concrete wall 82 capable of penetrating into this
device through openings (not shown), for maintenance of the device
shown in FIGS. 4 and 5.
* * * * *